The invention relates to an electronic data input keyboard comprising keys provided with conductive contacts, these keys being associated with crosspoints of a conductor matrix consisting of a number of column conductors and a number of row conductors and coupling, upon actuation, each time one of the column conductors to one of the row conductors via an uncoupling element which is connected in series with the contact, the conductors of one matrix coordinate (for example, column conductors) successively receiving a scanning pulse from a control circuit, the conductors of the other matrix coordinate (for example, row conductors) being interrogated for the occurrence of the scanning pulse by means of an evaluation circuit.
Such a keyboard is generally known, for example from the magazine "Computer design", October 1982, pages 137-142.
This article mentions that contemporary data input keyboards generally require a feature which is referred to as "n-key rollover". This is to be understood as the capability of the keyboard to output always the appropriate electrical signal when a key is depressed, regardless whether any previously depressed keys have meanwhile been released or not. The symbol "N" serves to indicate that an arbitrary number of (N) keys can remain depressed without the operation of the (N+1).sup.th key being disturbed. Such keyboard behavior is presently considered an absolute necessity of fast, ambidextrous input, because in given sequences and beyond a given speed the keys are struck faster than they are released, so that temporarily several keys are simultaneously in the depressed state; it being unacceptable for errors to occur in the recognition of the sequence in which the keys have been actuated.
FIG. 1 shows the circuit diagram of the known device. This Figure shows, by way of example, a small matrix which comprises four columns S1 to S4 and four rows Z1 to Z4. To each of the sixteen crosspoints there may be connected a key switch which connects, in its closed condition, the relevant column conductor to the associated row conductor through a diode which serves as an uncoupling element. In FIG. 1 three closed switches are shown by way of example in the form of diodes 22, 24 and 42. The contacts at the other crosspoints of the matrix are assumed to be open and have been omitted for the sake of clarity.
The electronic control system consists of a control section ST which cyclically applies a scanning pulse to the column conductors and an interrogation section A which monitors all row conductors for the appearance of the scanning pulse. Nowadays the control section and the interrogation section are preferably combined in an integrated microprocessor which also performs other functions.
When the control section ST applies the scanning pulse to the column conductor S1 as shown in FIG. 1, the interrogation section A will not detect a pulse potential on any of the four rows Z1 to Z4, because there if no conductive connection between S1 and Z1 to Z4.
When the control section ST applies the pulse to the column S2 at another instant, the interrogation section A will detect a pulse potential on the conductors Z2 and Z4 because corresponding conductive conductors are present, for example, the keys 22 and 24 are depressed.
Similarly, when the scanning pulse is applied to S4, a pulse potential will be detected on Z2 because the existing connection, for example, the depressed key 42. If the diodes were absent, however, the pulse potential would now also appear on Z4, which is completely undesirable. This occurs because as seen from the Figure a conducting conductor would then extend from S4 to Z2 and therefrom to S2 and further to Z4. This means that the interrogation circuit reacts as if the key at S4, Z4 were also closed, though that is not the case. Such positions in a switch matrix are referred to as "phantoms". When three of a number of simultaneously depressed keys form the corner points of a rectangle, the fourth corner point always represents such phantom. The diodes prevent such phantoms, because the diode which is connected in series with the key 22 in the described embodiment presents influencing of the column S2.
Several possibilities exist as regards the construction of key switches comprising conductive contacts. An example of a conductive keyboard which is described in this article is formed by a membrane keyboard. Therein, the contact points are printed on plastics foils, together with the connection conductors, in the form of planar elements consisting of an electrically conductive material which is suitable for silkscreening. The contact spots of two foils are arranged so as to face one another and are separated by a spacer foil which is perforated at the area of each contact spot. When a contact spot is pressed, it contacts the contact spot on the oppositely arranged foil through the hole in the spacer foil.
In such a membrane keyboard, however, it is virtually impossible to connect diodes to the silkscreened conductor pattern on a plastics foil. Soldering is impossible because of the temperature sensitivity of the foil and the silkscreening paste. Bonding of chip diodes as customarily used for silkscreened conductor patterns on ceramic substrates is not feasible for several reasons (cumbersome, usable only on hard substrates, mechanically vulnerable). Crimping by means of metal clamps and the like is problematic because of lack of reliability and space requirements.
Previous attempts have been made to provide an n-key rollover-type behaviour for such keyboards without diodes by distributing the individual keys of a field across the matrix so that the probability of occurrence of phantoms was minimized. Furthermore, it was attempted to control the phantom situations still occurring by a suitable composition of the microprogram of the electronic control system, i.e. by intermediate storage of the coordinates of the keys invovled in a phantom formation until the situation is resolved by the release of one of the relevant keys.
However, these steps could not ensure that a truly arbitrary number of keys in arbitrary positions on the keyboard could be simultaneously depressed in a rollover sequence without ever giving rise to errors. Such keyboards have an n-key rollover behaviour such that the number N fluctuates in dependence of the keys depressed and can decrease to as low as 2 from case to case. This behaviour is in conflict with the ergonomic requirements.
Moreover, in such a keyboard all keys which must be depressed during the actuation of other keys (upper case/lower case and other multiple keys) must be removed from the matrix in order to be separately connected to the electronic control circuitry. If this is not done, irresolvable phantom situations will occur notably when several multiple keys are used. Besides the drawback of these separate connections, there is another problem when use is made of freely programmable keys which must also be programmable as multiple keys. This is because in such cases it is not known in advance which keys will be involved so that in an extreme case all programmable keys should be removed from the matrix in order to be separately connected. Considering the growing importance of programmable keys in modern data processing systems, this may be a prohibitive drawback.